Journal of the Society of Motion Picture Engineers (1930-1949)

The only system which seemed feasible would make use of a photovoltaic cell type of meter so arranged as to keep the projectionist continually informed as to the light output of the projectors. With this guide, he could maintain the light at the standard value by trimming the arc. A photovoltaic cell is practical because it maintains its calibration quite well if protected from heat, moisture and intense light. Since a great deal is known about the use of photovoltaic cells as light meters, design of such a system should be relatively simple. Accordingly, a projector light meter was built around a photovoltaic cell. In order to monitor the light actually reaching the screen, a piece of unsilvered, optically flat glass was placed in the light path at an angle of 45°. This threw a beam of light off to the side of the projector, amounting to approximately 10% of the total output. This was more than enough for any photovoltaic cell, and at the same time caused a loss of only 10% in the screen brightness. This could be compensated for by trimming the arc. Obviously, the glass had to be positioned between the arc and the film gate. The photocell was a Weston Photronic cell type RR, and the associated ammeter was a 0 to 20-/*a, 2500-ohm Weston meter. Since the light from the optical flat was far too much for the cell, a means of attenuating this light was necessary. A dense green glass was placed ahead of the cell in the first model. This cut down the light to a workable level, but permitted a great deal of infrared radiation to be transmitted. This infrared energy raised the temperature of the cell too high for stability, so an Aklo heat glass was added. This promptly cracked. Thus it was evident that another means of reducing the heat was necessary. Ventilation slots were cut into the casting holding the cell, and this helped some, but the Aklo glass still would not stand up. At this point the projectionists at the M-G-M laboratory, who had been informed of our project and had built a model of their own, thought of replacing the dense glass filter by a sheet of brass shim stock with pinholes. This solved the excess heat problem, since the infrared radiation was reduced as much as the light. The first Cinecolor model used a bakelite mounted photocell, but M-G-M used a metal-encased cell for conduction cooling. The M-G-M modifications resulted in a cell mounting which was only slightly warm to the touch, even after many hours of continuous operation. The Weston microammeter, with its 2500-ohm resistance, gave a fairly linear response when coupled to the type RR Photronic cell. Various devices were considered for improving the linearity, including shunt resistances, lower resistance ammeters, and other types of photocells; but they all required more light, and consequently would have placed more heat at the photocell. Since heat dissipation was the biggest problem of the project, it was decided to accept the slight nonlinearity. The only advantage to improving the linearity would be to eliminate the scale compression in the operating range and thus increase the sensitivity. With the present model of the meter, however, luminosity fluctuations can be kept within about 3% and this is considered good. Most of this fluctuation is due to the coarseness of the trim, not the accuracy of the meter. The location of the unit in the projector is shown in Fig. 1. This view shows the first Cinecolor model mounted in a Simplex projector, just above the framing knob. The rear end of the photocell, showing the bakelite casing, is seen together with the two wires leading to the microammeter. The ammeter is mounted on the wall of the projection booth just below the viewing Harry P. Brueggemann: Continuous Arc Projector Meter 41